1. Field of the Invention
The present invention relates to a flat panel display device, and more particular to, an organic electroluminescent device, which can enhance device reliability while allowing simplification of a manufacturing process, and a method for manufacturing the same.
2. Discussion of the Related Art
In recent years, there is an increasing demand for providing flat panel display devices occupying a small space according to an increase in size of a display device. As one of such flat panel display devices, an organic electroluminescent device also referred to as an organic light emitting diode (OLED) has been rapidly advanced in its manufacturing technique, and widened in application thereof.
The organic electroluminescent device is a device that comprises a first electrode as an electron injection electrode (cathode), a second electrode as a hole injection electrode (anode) and an organic light emitting layer formed between the first and second electrodes in which electrons and holes injected through the first and second electrodes are recombined to form pairs of exitons, so that the pairs of exitons release light when they are extinguished as a result of energy transition from an exited state to a ground state.
Since the organic electroluminescent device has a merit in terms of its driving voltage of 5˜10 V which is lower than that of a plasma display panel (PDP) or an inorganic electroluminescent display device, it has been actively investigated.
In addition, since the organic electroluminescent device has excellent characteristics such as wide viewing angle, high response speed, high contrast and the like, it can be applied to pixels of a graphic display, and pixels of a TV screen or a surface light source. In addition, the organic electroluminescent device can be formed on a flexible transparent substrate like a plastic substrate, and formed in a very compact and light structure. Furthermore, since the organic electroluminescent device exhibits good color reproduction, it is appropriate for a next generation flat panel display.
Furthermore, since the organic electroluminescent device does not require a backlight member generally used in a well-known liquid crystal display (LCD), it has low power consumption, and provides excellent color sensation.
Generally, the organic electroluminescent device is generally classified into a passive matrix type and an active matrix type according to its structure and driving method.
Unlike the passive matrix type, in the case where the active matrix type is adapted to emit light through a glass surface of a substrate (typically, known as a bottom emission manner), an increase in the size or the number of thin film transistors (TFT) causes a rapid reduction in aperture ratio, thereby making it difficult to use the organic electroluminescent device as the display device.
In order to solve the problem, a top emission manner has been suggested, in which light is emitted through a side opposite to the glass surface so as to allow the aperture ratio to be independent of the size or the number of TFTs.
The top emission type organic electroluminescent device comprises a reflection layer, an organic light emitting layer, and a transparent electrode layer sequentially formed in this order on a substrate having TFTs and a storage capacitor formed therein, such that, when light is emitted from the organic light emitting layer, it is reflected by the reflection layer, and then emitted to an outside through an opposite side of the substrate. As a result, the organic electroluminescent device of this type is prevented from having its aperture ratio lowered due to the TFTs.
A conventional method for manufacturing a top emission type active matrix organic EL device will be described with reference to the drawings.
FIGS. 1A to 1F are cross-sectional views illustrating manufacturing steps of the conventional top emission type active matrix organic EL device.
At first, referring to FIG. 1A, a thin film transistor (TFT) 12 is formed in pixel unit on a transparent substrate 11.
Specifically, after forming an amorphous silicon layer on the transparent substrate 11, laser is illuminated on the surface of the amorphous silicon layer to form a poly-silicon layer through melting and recrystallization of the amorphous silicon layer. Then, the poly-silicon layer is patterned to form an island shape via a photolithography and etching process to form a semiconductor layer 12a. 
Next, a gate insulation layer 12b is formed on the overall surface including the semiconductor layer 12a, and a metallic layer comprising, for example, chrome (Cr) is formed thereon, followed by forming a gate electrode 12c at a location corresponding to a central portion of the semiconductor layer 12a on the gate insulation layer 12b via the photolithography and etching process.
Then, p-type or n-type impurities are implanted into the semiconductor layer 12a using the gate electrode 12c as a mask, after which heating is performed for the purpose of activating the implanted impurities, thereby forming a source electrode 12d and a drain electrode 12e in the semiconductor layer 12a. As a result, each of the TFTs 12 is completely formed.
After a first insulation layer 13 is formed on the overall surface including the TFTs 12, a contact 14 is formed so as to be connected with the source electrode 12d and the drain electrode 12e of each TFT 12 through the first insulation layer 13 and the gate insulation layer 12b, and a second insulation layer 15 is formed on the overall surface thereof.
Then, a flattening insulation layer 16 is formed on the second insulation layer 15 as shown in FIG. 1B, and selectively removed along with the second insulation layer 15 so as to expose the surface of the contact 14 connected with the drain electrode 12e via the photolithography and etching process, thereby forming a first contact hole 17.
Then, an anode electrode material 18 is deposited on the flattening insulation layer 16 and into the first contact hole 17 such that the first contact hole 17 is filled with the anode electrode material, as shown in FIG. 1C.
Next, as shown in FIG. 1D, an anode electrode 18a is divided in pixel unit by selectively removing the anode electrode material 18 via the photolithography and etching process, and then an insulation layer 21 is formed on a portion excluding a light emitting part.
Next, an organic EL layer 22 is formed on the overall surface, as shown in FIG. 1E, and a cathode electrode 23 is formed on the organic EL layer 22, as shown in FIG. 1F.
As a result, the conventional top emission type active matrix organic EL device is completed.
Meanwhile, adhesion between the anode electrodes 18a and the flattening insulation layer 16 is low. Accordingly, when removing the photoresist film used for the photolithography and etching process for dividing the anode electrodes, there is high possibility that the anode electrodes 18a are also separated from the flattening insulation layer 16.
The problems of the prior art described above will be set forth in detail with reference to the drawings hereinafter.
FIGS. 2A to 2D illustrate the problems which can arise when manufacturing the conventional organic EL device.
As shown in FIG. 1C described above, after depositing the anode electrode material 18, the photolithography and etching process is performed to allow the anode electrodes to be divided from each other in pixel units.
Specifically, as shown in FIG. 2A, after a photo-resist 19 is applied to the anode electrode material 18, a mask 20 having patterns to expose edges of each pixel is aligned on the transparent substrate 11, and the photo-resist 19 is exposed to light by illuminating the light towards the transparent substrate 11 from above the mask 20.
Then, the exposed portions of the photo-resist 19 are removed by stripping off the mask 20 and developing the photo-resist 19, as shown in FIG. 2B.
Next, as shown in FIG. 2C, after forming the anode electrodes 18a in pixel units by removing the anode electrode material 18 using the photo-resist 19 as a mask, the transparent substrate 11 is input to a stripper to remove the photo-resist 19, as shown in FIG. 2D.
At this time, separation of the anode electrodes 18a from the flattening insulation layer 16 occurs due to low adhesion between the anode electrodes 18a and the flattening insulation layer 16. As a result, device reliability is significantly deteriorated.